It depends on your actions, common sense and how you apply what you know. A teacher is smart and a construction worker is smart. Everyone has their good areas and bad areas. The politician knows about the world and foreign policy while a welder knows about how to make a lot of our fancy modern tech that we see everywhere.

I gather you're referring to smarts as a package concept, not merely intelligence, cleverness, creativity, or foresight on their own?

Usually, you can really get a great sense of how "smart" someone is by how well they solve issues on the fly; so the next time the shit hits the fan, see how a particular person recovers and takes ownership. Being able to connect things that may not be apparent cohorts is a good indicator, also.

The more adaptive you are, the smarter you will be percieved by others.

You can assume a person is stupid when they say shit like: "I thought Pleasantville was a village." Yes, that's a major deduction in intellectual points. It's also comments like that that make some people want to slap them. lol

I have no idea. People can smart if they always study like the Chinese people. They'd study like 7 days a week before their tests or exams. I'm telling you, if you study a lot. You'll be smart as hell.

How do u decide someone is smart or not? asking for everyone's idea ^^

Many answers in this thread already, more or less serious ones.
Deciding whether a person is smart or not is very crucial for entrance exams to school, selecting people to work for companies etc.
Although there is no method of judging intelligence that is 100% accurate and works on everyone. Every method has its strong and weak points. Most methods only tell like this person seems smart here but not there in these situations and contexts, which means they may be smart in other situations we haven't tested yet. Testing a persons intelligence in every situation and context takes too much time and effort not realistic nor possible.

The interesting thing to analyze and define is what the very word smart/intelligence really means.
It is pretty obvious there is various ways to see intelligence as being very good or like a master at one or many things.
But then we reach another issue, how do you define very good or master at anything? A very easy way with many faults, is to give a person a test on a subject with the multiple choices type and a max score of 100 points. If the victim gets 100 points you can say that person is smart due to the test she cleared. It doesn't tell more than that person could memorize a lot of data and with that pick the most accurate answers on the answer sheet within a short time period. Memorizing isn't that great of an ability in these days. It is easier to use a computer and notebooks to store data on or search though Google than to remember every single word in a few books.
It is way harder to search through vague texts in order to answer vague questions without clear answers compared to just find data and input it on paper that demands it to be exactly the same way from the text. A computer can do that much faster than a human anyway, and still you don't call the computer for "smart".
From much experience and from many books and research on learning and intelligence, i believe that people are good at different things and weak at others. No one is weak in everything. Nor do i believe that a person can master a subject that expands and change periodically without clear, obvious and one sided answers and facts.
Interesting is that humans and living beings can practice and get better at things the previously wasn't so good at. Even becoming great at it is possible but can take much longer time.
In that sense many claim that people who learns fast and remember much for long afterwards are smart. But such people tend to have weakness despite that, for example they can just memorize data without knowing what it means, not understanding problems within the text, can't talk about its contents with different words etc. So the time factor isn't that important rather than making the goals and purposes clear enough within reasonable limits like being able to communicate well enough for people to understand or grasp the core content of a book and summarize it with a few words.

On a social level, intelligence can be and often used as a mean to distinguish between people that belongs to your social circle or not, more or less close to you.
But is more simple in practice as judging people interests and hobbies as a sort of intelligence. For example Lina likes tennis and is good at. She see another person who plays tennis and accepts that person and grade her as "smart". But when Lina notice a person playing video games she sighs and say, "waste of time", the same as not smart. An easy example that people may refuse and say that taste is not connected to intelligence. That may be true, but not for everyone and everything.
It gets more complicated if Lina would tolerate the gamer and still think tennis is better than video games. If she did, i would say she is more intelligent than what she would be in the first example. Still this doesn't mean she is good with school and alike.

So deciding if someone is smart takes time and is complicated. It may change over time as well.
I hardly believe anyone would spend all her free time on judging the intelligence level of every person she meet and use a very long and complicated formula. Even if a person would try to, i think the method of grading would be quite simple in order to save time and energy and in that way be able to grade people way smoother and faster. With that she can grade many than a selected few.

I personally can judge people once in a while. The way i do it takes time and effort. For example one thing is that i dont like people that always writes some few words or sentences in replies and posts no matter what the length of the content or complex the matter is.
Like this: My question - how can you solve the ongoing conflict between Israel and Palestine? The reply from a person -
Just build asphalt over all the land there and they wont fight anymore. It is a very simple answer without much thinking needed nor writing. If a person writes a reply like that i would think that there is some underlying reason, like one of the following:
the person dont like writing, is lazy, dont care about the conflict, dont like talking or discussions, have little free time, have to send many short messages to all the hundred of friends he has etc.

However, i tolerate and accept that people may write short texts and such in posts, pm's and so on but i dont like it, unless it fits the situation like it is a small matter we talk about or just a simple question and reply context.

smart or not? hmm. i decide if peeps are smart bu lookin at how they are able to logically ponder and calculate the best possible solutions to any problem. peeps who get good grades arent neccessarily bright/smarty it just means that they can study or is good at that paticular stuff.

Depends on what you mean by smart . If you mean book smart , of course you can always tell by grades and what not . Their academic scores should be high . Doctors , lawyers , teachers , etc , are part of this area. However , getting good grades can mean you're book smart , but you can fail in other parts of life , and I think people who are smart all around are the ones that go far in life , therefore making them the people that I consider smart . This means you don't have to be book smart to be smart . Having wonderful ideas and a large imagination , then applying those skills to different parts in your life is intelligent .